FIG. 11 schematically illustrates a filter element used heretofore as a dust removing apparatus for combustion gas and gas produced by gasifying petrified fuel such as coal. Referring to FIG. 11, a prior art filter is described for the removal of soot dust contained in combustion exhaust gas as an example.
In FIG. 11, a pillar-shaped filter element 011 having a substantially square cross-section is accommodated in a filter element mounting frame 012 made of metal or ceramics and both ends of the element are held by metal fittings 013. Further, filling material 014 such as felt is filled into a gap between the filter element 011 and the frame 012 to seal therebetween.
Dirty gas 21 containing soot dust in combustion exhaust gas is passed through the filter element 011 to be filtered therein, so that soot dust in the exhaust gas is removed and purified gas 22 is discharged.
As shown in FIG. 13, a plurality of the filter elements are collected and are accommodated into an outer frame 023 to be used as a pack.
Further, FIG. 14 illustrates a conventional dust removing apparatus using a honeycomb pack in which a unit number of honeycomb type filter elements are collected to form a unit lot. The apparatus includes thin paper-like seal packings 02 disposed on the side receiving dirty gas and on the side emitting purified gas of filter elements 01 to hold the seal packings 02 between contact surfaces of the filter elements 01. The seal packings 02 are wound on all of the filter elements 01 arranged into a plurality of row and columns and all of the filter elements are finally fastened by a honeycomb pack frame externally. As shown in FIG. 15, the whole outer periphery of the filter elements 01 is wound at inlet and outlet sides thereof by seal packings 04 and is fastened by the honeycomb pack frame 03 to effect sealing in the honeycomb pack frame 03.
FIG. 16 schematically illustrates an order of assembling such a honeycomb pack. More particularly, first of all, as shown in FIG. 16(A), a honeycomb pack bottom frame 05 and a center partition plate 06 are held by a jig 09. Then, as shown in FIG. 16(B), the filter elements 01 each having the above paper-like seal packings 02 which are previously wound on both ends of the filter element are piled up. In this case, seal packings 04 are disposed on outer end portions of the bottom partition plate 05 and the center partition plate 06.
After the filter elements have been piled up completely, seal packings 04 are attached on the whole outer peripheries of the piled filter elements as shown in FIG. 16(C). Then, as shown in FIG. 16(D), an upstream lattice frame 07 is fitted onto the pack on the dirty gas side thereof and a downstream lattice frame 08 is fitted onto the pack on the purified gas side thereof. Finally, as shown in FIG. 16(E), the pack frame 03 disposed on the upper surface is fastened to complete the pack.
In the dust removing apparatus fabricated by the above-described processes, the dimensional accuracy of the filter element is varied and, accordingly, adjustment of clearance is performed by the piling method of the packings. Thus, the sealing performance is influenced by the technical skill of operators who perform the assembling of the pack as described above.
In the conventional filter element shown in FIG. 11, when the temperature of the dirty gas containing the soot dust is high, size difference between the filter element 011 and the filter element mounting frame 012 in which the filter element is accommodated is increased due to the high temperature. The increased size difference cannot be covered by the resilient width of the filling material 014 such as felt filled therebetween to thereby form the clearance. Accordingly, as shown by arrow a of FIG. 12, gas is leaked through the clearance and part of soot dust cannot be filtered by the filter element 011 to thereby reduce the soot dust collection factor.
For example, when the filter element 011 is formed into a square pillar having one side of 200 mm and a temperature of the dirty gas 21 is 800.degree. C. on condition that the filter element 011 is made of ceramics and has the coefficient of thermal expansion of 1 .times.10.sup.-6 /.degree. C., and the filter element mounting frame 012 is made of metal and has the coefficient of thermal expansion of 18 .times.10.sup.-6 /.degree. C., an increase .DELTA.Hcl of the clearance between the filter element 011 and the filter element mounting frame 012 in the gas flowing direction upon the normal assembling (20 .degree. C.) and the high-temperature operation is given by EQU .DELTA.Hcl =200.times.(18-1).times.10.sup.-6 .times.(800-20) =2.652 mm
Further, in the conventional dust removing apparatus shown in FIG. 14, the seal packings are wound on the gas inlet side surfaces and the gas outlet side surfaces of the filter elements, and the filter elements are piled up vertically and horizontally. In addition, the packings are provided under the honeycomb pack frame and the frame is fastened to effect complete sealing. Accordingly, the assembling of the apparatus is apt to be affected by the operator's assembling ability. Also, errors in the element dimension and a problem of the extension difference occur at the high temperature. In other words, since the honeycomb pack frame made of metal expands a larger amount as compared with the element made of ceramics the occurrence of looseness is unavoidable even if the pack is sufficiently fastened. Further, in the case of the class of 1,000 mm H.sub.2 O in which a differential pressure between the inlet side and the outlet side of gas is very large, there occurs a phenomenon that the seal packing on the inlet side is shifted inside gradually. Accordingly, the sealing of the honeycomb pack of the dust removing apparatus has such problems and its improvement there of is needed in the art.